This invention relates generally to the field of optical information processing and more particularly to a method and apparatus for ultrasonic laser testing. More specifically, the present invention provides an improved laser source for generation of ultrasound. Even more particularly, the laser source of the present invention can significantly improve the characteristics of the ultrasound generated by modulating the optical and temporal characteristics of the generation laser to optimize the characteristics of the generated ultrasound.
In recent years, the use of advanced composite structures has experienced tremendous growth in the aerospace, automotive, and many other commercial industries. While composite materials offer significant improvements in performance, they require strict quality control procedures in the manufacturing processes. Specifically, non-destructive evaluation (xe2x80x9cNDExe2x80x9d) methods are required to assess the structural integrity of composite structures, for example, to detect inclusions, delaminations and porosities. Conventional NDE methods are very slow, labor-intensive, and costly. As a result, testing procedures adversely increase the manufacturing costs associated with composite structures. Various methods and apparatuses have been proposed to assess the structural integrity of composite structures. One method to discloses the use of a pulsed laser beam for generating ultrasound on a work piece and a second pulsed laser beam for detecting the ultrasound. Phase modulated light from the second laser beam is then demodulated to obtain a signal representative of the ultrasonic motion at the surface of the work piece. A disadvantage associated with this approach is that the first pulsed laser beam is not optimized for the generation of ultrasound in the workplace.
Prior solutions describe operable techniques for optically detecting transient motion from a scattering surface, which techniques are useful for ultrasonic composite materials non-destructive test and evaluation, these techniques have numerous failings.
Known techniques provide the ability to perform common mode noise cancellation. By using a single laser signal, the known techniques cannot perform differential mode operation. An adverse consequence of this characteristic is the inability to use known ultrasonic systems in factory or industrial settings where ambient light noise levels exceed certain threshold levels. This problem prevents the proper detection of scatter signals from the composite materials.
Another limitation associated with none ultrasonic test and evaluation techniques relates to their broad scanning approaches to determine the existence of a transient, thereby indicating a defect. Because broad scanning occurs, both the amount of data is excessive and the degree of accuracy is lower.
Another limitation associated with the known systems relates to their ability to process ultrasonic data in real-time. This limitation makes such systems only marginally useful for testing and evaluating composite materials.
Other limitations associated with existing systems relate to general inflexibility of such systems, which may hold all distances low, result in small depth of field performance and only minimal extraction of information from the back scattered signals. These limitations make industrial application of the ultrasonic testing method generally impractical.
Therefore, it would be desirable for a new method and apparatus for ultrasonic laser testing that overcomes the disadvantages and deficiencies of the prior art.
In light of the above, a need exists for a system and method that generates a desired frequency content in laser-generated ultrasonic waves. The present invention provides a system and method for generating laser radiation with a tunable wavelength and temporal shape that substantially eliminates or reduces disadvantages and problems associated with previously developed systems and methods for laser inspection.
More specifically, the present invention provides a system for generating laser radiation with a tunable wavelength and temporal shape. This system includes a first pulsed laser to generate a first pulse laser beam. The first pulse laser beam is produced having a first wavelength. A wavelength shifting device shifts the first wavelength of the first laser beam to a second wavelength. A modulator then modulates the laser beam having the second wavelength.
In another embodiment, the laser radiation with a tunable wavelength is utilized to generate ultrasonic displacements at a remote target to be inspected. This involves generating a first laser beam having a first wavelength. A wavelength shifting device then shifts the first wavelength of the first laser beam to a second wavelength. An optical modulator is used to modulate the second wavelength laser beam""s temporal shape. This laser beam is then used to generate ultrasonic displacements at a remote target wherein the ultrasonic displacements at the remote target have a desired frequency content and are used for NDE of the remote target.
The present invention provides an important technical advantage in that a laser-generated ultrasonic wave can be formed with a desired frequency content and temporal shape. Thus, for certain materials that require specific ranges of acoustic frequency to adequately perform a non-destructive evaluation of the material, an optimal set of ultrasonic displacements can be determined. In turn, the wavelength shifting device is used to shift the first wavelength to a wavelength, which generates the optimal laser pulse to produce the desired range of acoustic energy in the ultrasonic wave. Therefore, depending on the thickness or composition of the materials, the desired ultrasonic displacements can be generated to produce the best resolution for inspection.
Additionally, the attenuation of the ultrasound can be controlled, allowing a user to optimize the inspection techniques for the defects to be searched for. Furthermore, by understanding the attenuation characteristics of the ultrasound generated in the target, the scanning technique can be optimized based on these characteristics to reduce or eliminate over sampling and therefore increase the speed and efficiency of the inspection which decreasing cost.
The present invention provides another technical advantage in that a system is provided for flexible, accurate, and cost-effective methods for inspecting complex composite structures. The present invention is able to optimize a scan and test a large size composite structure based on empirical data associated with the composite structure or data modeled on the composition of the structure.